Validation of a novel approach to predict total tract NDF digestibility in dairy cattle using an in vitro assay

Three experiments were conducted to compare estimates of fiber digestion predicted from in vitro fiber digestibility analysis of diets to fiber digestion measured in lactating cows fed the same diets.;In the first experiment, an in vivo study was performed to validate an in vitro procedure that predicts rate of fiber digestion and total tract fiber digestibility (TTNDFD). Two corn silages differed in fiber digestibility (high and low) were used in this trial. Two diets were formulated and analyzed using the in vitro assay to predict the TTNDFD and rumen potentially digestible NDF (pdNDF) and rate of digestion (kd). Similar diets were fed to eight ruminally cannulated multiparous dairy cows at restricted intake (approximately 90% of ad libitum intake) and at ad libitum intake. The in vitro TTNDFD test predicted total tract NDF digestibility of diets similar to digestibility observed in vivo, but for Low fiber digestibility corn silage (LFDCS) diets, this test underestimated the digestibility compared to in vivo. These results suggest that the in vitro TTNDFD model could be used to predict rate of fiber digestion and NDF digestibility for dairy cows.;In a second trial, the effect of altering ratios of corn silage and alfalfa silage on fiber digestion were evaluated. The two forages had similar NDF composition, but fiber in the corn silage contained less indigestible NDF (iNDF) than the alfalfa silage. Four diets were formulated containing different DM proportions of alfalfa and corn silage, and fed to eight ruminally-cannulated dairy cows. Total tract fiber digestibility and fiber digestion kinetic parameters observed in vivo were compared to the values predicted by the in vitro assay and model. Total tract NDF digestibility coefficients were similar for the in vitro and in vivo method. As the proportion of dietary alfalfa increased, the digestibility of NDF increased. The rate of digestion of pdNDF predicted from the in vitro assay was also similar to what was observed. Results suggest that the in vitro TTNDFD model could be used to predict rate of fiber digestion and NDF digestibility for lactating dairy cattle.;The objective of the third experiment was to validate an in vitro test to predict total tract fiber digestibility in lactating cows by an in vitro assay. Twenty-one diets fed to lactating cows in seven different studies were used for this trial. Fiber digestibility measured in vivo was similar to values obtained from the in vitro assay. This in vitro approach can predict values of fiber digestibility similar to values directly measured in vivo.